1. Field of the Invention
The present invention relates to a method of cleaning a film forming apparatus for forming a film on a workpiece, such as a semiconductor wafer, a cleaning system for carrying out the same, and a film forming system.
2. Description of the Related Art
A semiconductor wafer is subjected to various processes including a film forming process and an etching process to fabricate a semiconductor integrated circuit thereon. When simultaneously forming films respectively on a plurality of semiconductor wafers by a CVD system, the semiconductor wafers are arranged at equal intervals on a wafer boat, the wafer boat holding the semiconductor wafers is placed in an evacuated vacuum processing vessel, the semiconductor wafers are heated at a predetermined temperature, and then process gases for forming a film is supplied into the processing vessel to deposit a substance produced by the decomposition or the chemical reaction of the process gases in a film on the semiconductor wafers.
When thus forming the films on the surfaces of the semiconductor wafers, unnecessary films are deposited on surfaces not intended to be coated with any film, such as the inner surface of the processing vessel and the surface of the wafer boat. Such unnecessary films deposited on such surfaces not intended to be coated with any film come off in floating particles and the particles adhere to films deposited on the semiconductor wafers to cause defects in semiconductor integrated circuits fabricated on the semiconductor wafers. Therefore, the vacuum processing vessel is cleaned periodically or at indeterminate intervals.
Cleaning is needed not only by the hot-wall processing vessels of batch type LP-CVD systems (low-pressure chemical vapor deposition systems), but also by single wafer film forming systems.
It has been a common practice to clean the hot-wall processing vessel of an LP-CVD system for periodic cleaning by a wet cleaning method using a chemical liquid regardless of the type of the processing vessel, either a vertical type or a horizontal type.
The wet cleaning method, however, needs time for maintenance, increases downtime and has the following problems in view of productivity.
1 Removal and installation of the quartz reaction tube requires complicated, troublesome work.
2 Work for removing the quartz reaction tube must be suspended for a long time necessary for cooling the heater.
3 Work for the leakage inspection and the temperature profile inspection of the quarts reaction tube takes much time.
A most efficient means for solving such problems is removing films deposited by CVD on the surfaces of the LP-CVD system by on-site cleaning without disassembling the LP-CVD system, that is, making the LP-CVD system clean itself. Cleaning methods using ClF.sub.3 gas as an etching gas for cleaning a hot-wall type LP-CVD system are proposed in JP-A Nos. 3-31479, 4-155827 and 6-151396. These cleaning methods supply a cleaning gas containing ClF.sub.3 gas into the processing vessel to remove unnecessary films deposited on the wafer boat and the inner surface of the processing vessel.
Determination of a cleaning operation terminating point where the cleaning operation is to be terminated is important in carrying out the cleaning operation. The unnecessary films cannot satisfactorily be etched and fragments of the unnecessary films will remain on the wafer boat and the inner surface of the processing vessel if the cleaning time is not long enough to remove all the unnecessary films completely. The quartz wafer boat and the processing vessel will react with and will be etched by the highly reactive ClF.sub.3 gas and fine particles will be formed on the surfaces of the quartz wafer boat and the processing vessel if the cleaning time is excessively long.
Accordingly, it has been a common practice to subject a sample wafer similar to in-process wafers as a monitor to a film forming process together with the in-process wafers. A cleaning operation is started upon the increase of the cumulative amount of a film deposited on the sample wafer to a predetermined amount, and the cleaning operation is terminated upon the elapse of a predetermined cleaning time necessary for removing the predetermined amount of the film estimated on the basis of a cleaning rate determined beforehand.
However, the cumulative amount of the film deposited on the sample wafer does not necessarily coincide with and, in most cases, is different from the amount of the film deposited on the wafer boat or the inner surface of the processing vessel. Process conditions are not necessarily fixed; for example, process temperature is changed, and a dopant is added to or any dopant is not added to polysilicon films. Consequently, a plurality of layers of different materials are deposited on the wafer boat and the inner surface of the processing vessel. Since films of different materials are etched at different etch rates, respectively, it is considerably difficult to determine exactly a cleaning operation terminating point where the cleaning operation is to be terminated on the basis of the cleaning rate (etch rate) determined beforehand.
Therefore, the cleaning operation is continued for a cleaning time slightly shorter than that may be necessary, the cleaning operation is interrupted to examine the remaining unnecessary film and then the cleaning operation is resumed and continued for an additional cleaning time determined through the examination of the remaining unnecessary film. Thus the cleaning operation takes a long cleaning time.